When Do Floating Shelves Not Float?

A question commonly asked in the shelving industry is "How do I get my floating shelves to line up with my studs?" I guess the simplest answer is, "you can't". The reason for this issue lies in the fact that neither the studs nor the fixing points of the floating shelves can move. They are both fixed at a predetermined distance. This was never a problem when buildings were constructed in solid brick and blockwork. Nothing wrong with them. They're great. But they are just not so common anymore as we all move to the speed and cost savings of cavity stud walls. 

So when your house's structural elements and your shelving system's positions are fixed at different positions, you have a problem, as the two widths will never align perfectly.

So, a floating shelf that adjusts to suit the width of your studs? That would make life... simpler.